Field of the Invention
The present invention relates to image processing technology for sharpening processing of an image generated by an image pickup apparatus, such as a digital camera.
Description of the Related Art
The unsharp mask processing (sharpening processing) is known as processing of improving the sharpness of an (input) image generated through capturing by an image pickup apparatus, such as a digital camera. The unsharp mask processing realizes sharpening by adding to the input image, data of a difference between an original input image and a blurred image made by applying an unsharp mask to the input image. The unsharp mask uses an image blurring filter, such as a smoothing filter, and an image area having a larger difference between the input image and the blurred image can be more effectively sharpened.
Japanese Patent Laid-Open No. (“JP”) 2010-81263 discloses a method for applying an asymmetrical one-dimensional filter to a pixel signal row arranged in an image height direction (meridional direction) and for correcting a false color and unnecessary coloring caused by a point spread function (“PSF”) of an optical system.
The conventional unsharp mask processing uses a rotationally symmetrical filter for the unsharp mask. Thus, it cannot correctly sharpen an input image that was deteriorated by a PSF having a complicated shape, such as an asymmetrical aberration and a sagittal halo. This means that an attempt to correct an aberration in an azimuth direction in which a large amount of aberration occurs causes undershoot in an azimuth direction in which a small amount of aberration occurs, and an attempt to restrain the undershoot cannot fully correct the aberration.
The method disclosed in JP 2010-81263 considers the asymmetry and changes in the image height of the PSF but these changes are those in the image height direction of the PSF. The correction filter used for this method is also a one-dimensional filter, and cannot improve the asymmetry in a direction other than the image height direction. Moreover, the method disclosed in JP 2010-81263 adjusts the asymmetry of the filter by the number of minus tap coefficients for the filter. When the actual asymmetry is different from the blur caused by the PSF of the optical system, sufficient sharpening is unavailable even in the image height direction.
In the rotationally symmetrical filter, the center position of the filter accords with the center of gravity position and the peak position of the coefficients of the filter. However, this is not always the case in the rotationally asymmetrical filter. When the relationship among them changes, the sharpening effect may reduce or a problem may occur.